渤海湾盆地桩海地区古近系沙河街组一段-二段混积岩优质储层特征与控制因素

Characteristics and controlling factors of high-quality reservoirs of mixed siliciclastic-carbonate sediments in the 1st to 2nd members of the Paleogene Shahejie Formation,Zhuanghai area,Bohai Bay Basin

渤海湾盆地桩海地区古近系沙河街组一段-二段广泛发育混积岩。近年来桩海地区混积岩油气勘探不断取得突破,已逐渐成为古近系重要的勘探目标。利用岩心观察、薄片鉴定、扫描电镜观察及孔-渗分析等技术手段,研究了桩海地区混积岩储层特征及优质储层控制因素。结果表明:混积岩中的陆源碎屑颗粒多为近源沉积,岩石成分成熟度低。来自长堤凸起的岩屑主要为中-酸性喷出岩岩屑,来自埕岛凸起的岩屑主要为石英岩等变质岩岩屑。用陆源碎屑、化学成因泥晶碳酸盐、生物成因碳酸盐颗粒为三端元,将混积岩划分为以陆源碎屑为主的混积岩(陆源碎屑含量在50%~85%)、以化学沉淀碳酸盐为主的混积岩(化学沉淀碳酸盐含量在50%~90%)、正混积岩(三组分均<50%)、以生物碎屑为主的混积岩4类。其中以生物碎屑为主的混积岩储层物性最好,平均孔隙度为15%~25%,储集空间主要为原生粒间孔和生物碎屑孔。泥晶化作用和环边亮晶胶结作用在以生物碎屑为主的混积岩中发育程度最高,二者形成的刚性格架使得原生孔隙得以保留。大气水淋滤和有机酸充注有利于储层次生孔隙的发育。碳酸盐嵌晶胶结在正混积岩中最发育,持续的压实作用对以化学沉淀碳酸盐为主的混积岩影响最大。沉积期较高的岩石结构成熟度和较高的生物碎屑含量,准同生期泥晶包壳的发育、大气淡水的淋滤以及环边亮晶胶结物的发育,埋藏成岩期发生的较强有机酸溶蚀是优质混积岩储层形成的有利条件。

Mixed siliciclastic-carbonate sediments(MSCSs)occur extensively in the 1st to 2nd members of the Shahejie Formation(Es1-2),Zhuanghai area,Bohai Bay Basin.Over recent years,breakthroughs have been constantly achieved in hydrocarbon exploration in the MSCSs in the Zhuanghai area,making these rocks gradually become significant exploration targets of the Paleogene strata.Using techniques such as core observation,thin section observation,scanning electron microscopy(SEM),and porosity and permeability analyses,we investigate the characteristics of MSCS reservoirs in the Zhuanghai area and the factors controlling the formation of high-quality reservoirs within.The results indicate that terrigenous clastic grains in the MSCSs in the Zhuanghai area are dominated by proximal sediment and exhibit lowmaturity in rock component.Rock debris originating from the Changdi uplift is dominated by intermediate-acidic extrusive rock detritus,while that from the Chengdao uplift consists primarily of the detritus of metamorphic rocks such as quartzites.Using terrigenous clasts,chemogenic micritic carbonates,and biogenic carbonate grains as three end-members,the MSCSs in the Zhuanghai area are categorized into four types:terrigenous clast-dominated MSCSs(terrigenous clast content:50%to 85%),chemically precipitated carbonate-dominated MSCSs(chemically precipitated carbonate content:50%to 90%),ortho-MSCSs(contents of the three components:all less than 50%),and bioclast-dominated MSCSs.The reservoirs of bioclast-dominated MSCSs exhibit the most favorable physical properties among others,with an average porosity ranging from 15%to 25%and storage spaces composed largely of primary intergranular and bioclastic pores.Micritization and the sparry cementation on the rims are the most developed in the bioclast-dominated MSCSs,with the rigid framework formed by both allowing for the preservation of primary pores.Meteoric water leaching and organic acid charging are conducive to the formation of secondary pores in the MSCS reservoirs.Carbonate cementation with poikilitic textures is the most developed in the ortho-MSCSs,while continuous compaction produces the most significant impacts on the chemically precipitated carbonate-dominated MSCSs.Favorable conditions for the formation of high-quality MSCS reservoirs include(1)the high rock-texture maturity and bioclastic content in the depositional period;(2)the development of micrite coats,meteoric water leaching,and the formation of the sparry cements around the rims during the penecontemporaneous period,and(3)strong organic acid dissolution during the burial and diagenetic period.